For this study methods of immature embryo cultivation and induced mutagenesis were used. The application of these methods in combination leads to the increase of plant cell mutation frequency up to 10-3 (Sidorov and Maliga, Mol. Gen. Genet. 186:328-332, 1982). The experiment was conducted with the following inbred lines: 346, 346wx, 502 and 502wx. Immature embryos at age 15-16 days were subjected to gamma rays applied at 0.16 Gr/second from the Co 60 radiation source. The treatments were 2, 4, 8 and 12 Gr. After irradiation embryos were planted on MS nutrient medium (Murashige and Skoog, Physiol. Plant. 15:473-497, 1962) supplemented with 2 mg/l 2,4-D. Analysis of variance has been carried out to determine the effect of genotype and gamma ray treatment for callus formation and somatic embryogenesis.

It should be noted that in the studied material different types of callus were observed. White colour callus of dense consistency was defined as type 1 and yellow colour callus of friable consistency (possessing rhizogenic ability) was defined as type 2. All variants of lines 346, 346wx and control variants of lines 502, 502wx proved to be type 1 callus-producing. In contrast, the irradiated embryos of lines 502 and 502wx produced type 2 callus formation. The results of the analysis of variance have shown that the type of callus formation was influenced by genotype, gamma rays and their interaction (Figures 1 and 2). During further cultivation the type 2 callus formation (for line 502wx) was suppressed in all variants of gamma ray treatment. A similar effect (for line 502) was induced by 8 and 12 Gr doses.

According to the results of the analysis of variance of somatic embryogenesis frequency significant differences have been determined between genotypes studied. The lines 346 and 502 have demonstrated a higher somatic embryogenesis frequency than their waxy counterparts (Figure 3), however, inbred lines 502 and 502wx have been more radiosensitive in comparison to lines 346 and 346wx. It should be emphasized that the irradiated embryos of all genotypes studied had a lower somatic embryogenesis frequency than control variants (Figure 4).

In general, the results obtained indicate that the influence of gamma radiation on callus formation and somatic embryogenesis was reliable (on all significance levels) and resulted in decreasing of callusogenesis and somatic embryogenesis frequency.

Figure 1. The influence of genotype and gamma rays on type 1 callus formation. 1-6 346wx; 7-12 346; 13-18 502wx; 19-24 502.

Figure 2. The influence of genotype and gamma rays on type 2 callus formation. 1-6 346wx; 7-12 346; 13-18 502wx; 19-24 502.

Figure 3. The influence of genotype on somatic embryogenesis. 1 346wx; 2 346; 3 502wx; 4 502.

Figure 4. The influence of gamma rays on somatic embryogenesis. 1 control; 2 2Gr; 3 4Gr; 4 8Gr; 5 12Gr; 6 irradiated seeds.

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